1. Field of the Invention
The invention relates to a process for depositing polycrystalline silicon.
2. Description of the Related Art
Polycrystalline silicon (polysilicon) serves as a starting material for production of monocrystalline silicon for semiconductors by the Czochralski (CZ) or zone melting (FZ) processes, and for production of mono- or polycrystalline silicon by various pulling and casting processes for production of solar cells for photovoltaics.
Polycrystalline silicon is generally produced batchwise in the Siemens process. This involves thermally decomposing a silicon-containing reaction gas or reducing it by means of hydrogen, and depositing it as high-purity silicon on thin filament rods of silicon, called “thin rods” or “cores”.
The silicon-containing component of the reaction gas is generally monosilane or a halosilane of the general composition SiHnX4-n (n=0, 1, 2, 3; X=Cl, Br, I). It is preferably a chlorosilane or chlorosilane mixture, more preferably trichlorosilane. Predominantly SiH4 or SiHCl3 (trichlorosilane, TCS) is used in a mixture with hydrogen.
The Siemens process is performed in a deposition reactor. EP 2 077 252 A2 describes the typical construction of a reactor type used in the production of polysilicon. In the most commonly used embodiment, the deposition reactor comprises a metallic base plate and a coolable bell jar placed onto the base plate so as to form a reaction space within the bell jar. The usually bell-shaped deposition reactor has to be closeable in a gas-tight manner since the reaction gases are corrosive and, in a mixture with air, have a tendency to self-ignition or to explosion. The base plate is provided with one or more feed orifices and one or more removal orifices for the gaseous reaction gases, and with holders which help to hold the thin rods in the reaction space. In general, two adjacent rods are connected by a bridge at their free ends, opposite the held foot ends, to form a U-shaped support body. The U-shaped support bodies are heated up to the deposition temperature by direct passage of current and the reaction gas is supplied.
A typical procedure for deposition of polysilicon involves opening a shutoff valve for the reaction gas flowing to the reactor and a shutoff valve for the offgas flowing out of the reactor. The reaction gas flows through a feed orifice in the base plate into the closed deposition reactor. The silicon is deposited therein on the thin rods heated by direct passage of current. The hot offgas formed in the reactor leaves the reactor through a removal orifice in the base plate and can then be subjected to a processing operation, for example a condensation, or can be sent to a scrubber.
In the deposition of silicon, the halogen-containing silicon compounds, for example trichlorosilane, decompose from the gas phase on the surface of the heated thin rods. In the course of this, the diameter of the thin rods grows. After the attainment of a desired diameter, the deposition is ended and the polysilicon rods formed are cooled to room temperature.
After cooling the rods, the bell jar is opened and the rods are withdrawn with deinstallation aids for further processing. Subsequently, the bell jar and base plate of the reactor are cleaned and provided with new electrodes and thin rods for the next deposition batch. After the bell jar has been closed, the process for depositing the next batch of polysilicon is again performed as described.
From the time of opening of the reactor until the deinstallation of the batch deposited, the polysilicon rods are in contact with ambient media such as room air with the corresponding constituents of nitrogen, oxygen, moisture, but also impurities in the form of extraneous constituents present in the air (metals, nonmetals, gases). The opening also causes a possible exchange of gas between the reactor interior and the deposition room. In this case, reactants, products or even constituents which have already reacted fully or partly (e.g. HCl(g)) which remain in the reaction space after the deposition can enter into the ambient air through exchange of gas.
More particularly, this is the case for the penetration of moisture from the ambient air into the reactor. When moisture penetrates specifically into the feed and removal lines of the reactor, and when bell jar deposits are present (solids of main constituents remaining on the inside of the reactor after the deposition, containing the elements Si, Cl and O), halosilane residues, for example unconverted reaction gas, or halosilanes or polysilanes formed in the process, result in the formation of corrosive hydrogen halides, for example hydrogen chloride. These corrosive gases can escape from the deposition reactor into the production room and lead, for example, to corrosion on lines, fittings, and technical components therein.
Especially the hydrogen halide corrodes reactor components, including the feed and removal lines in the reactor. The corrosion process gives rise to damage in the form of rust formation on steel surfaces in, for example, components of the deposition plants (flanges, connections). The corrosion which has occurred firstly causes a change in the surface properties and, as a result, releases metal particles (for example steel and alloy constituents Fe, Cr, Ni, Mn, Zn, Ti, W), and also releases electrically active dopants such as boron, phosphorus, aluminum and arsenic. These substances are introduced into the silicon deposited to an increased degree in the subsequent deposition, particularly on commencement of deposition, and deposited on the rod surface of polysilicon rods present in the production space.
Particularly, corroded steel can lead to unwanted deposits on the rod surfaces when the deposition reactors are opened for batch changeover and deinstallation of the polycrystalline silicon rods. The release of, for example, iron and deposits thereof on the rod surface can lead to a reduction in the lifetime in the resulting product for the semiconductor or solar industry.
U.S. Pat. No. 7,927,571 (DE 102006037020 A1) discloses a method for the batch production of high purity polycrystalline silicon, in which an inert gas is fed through the supply line and the discharge line into the open reactor from the time when the deposition reactor is opened in order to extract the first substrate body with deposited silicon until the time when the reactor is closed in order to deposit silicon on the second substrate body.
GB 1532649 discloses a method of depositing polycrystalline silicon on a graphite surface, wherein the closed reactor is purged with an inert gas, for example argon, before commencement of the heating of the deposition surface or shortly before the deposition. This inert gas purging for purging of a closed reactor during the course of the process is effected for inertization or for avoidance of explosive gas mixtures (oxygen removal).
The feeding of inert gas into the reactor during the process or after the opening of the reactor, which is disclosed in the prior art, does not solve the problem of reactive depletion of the bell jar deposits. Nor is the problem of introduction of extraneous material into the reaction space and onto the rod surfaces in the operation of reactor opening remedied thereby.
US 2012/0100302 A1 discloses a method for producing polycrystalline silicon rods by deposition of silicon on at least one thin rod in a reactor, wherein, before the silicon deposition, hydrogen halide at a thin rod temperature of 400-1000° C. is introduced into the reactor containing at least one thin rod and is irradiated by means of UV light, as a result of which halogen and hydrogen radicals arise and the volatile halides and hydrides that form are removed from the reactor. This cleans the thin rod surface before the start of deposition. During the batch changeover or the installation of the thin rods, feed and removal lines and the bell jar are purged with inert gas (nitrogen) in the open state.